Exhaust gas diffuser for a gas turbine

ABSTRACT

An exhaust gas diffuser for a gas turbine generally includes an inner wall that extends along an axial centerline of the exhaust gas diffuser. An outer wall is coaxially aligned with the inner wall. The outer wall is radially separated from the inner wall so as to define a flow passage therebetween. An airfoil shaped strut is disposed in the flow passage. The strut extends between the inner and the outer walls. The strut includes a leading edge and a trailing edge positioned relative to a direction of flow through the flow passage. The leading edge and the trailing edge are tapered from the inner wall to the outer wall in the direction of flow through the passage.

FIELD OF THE INVENTION

The present invention generally involves an exhaust gas diffuser for a gas turbine. More specifically, the present invention describes a strut that reduces flow separation within the exhaust gas diffuser to improve the efficiency of the gas turbine.

BACKGROUND OF THE INVENTION

Gas turbines are widely used in industrial and power generation operations. A typical gas turbine includes a compressor section, a combustor downstream from the compressor section, and a turbine section downstream from the combustor. A working fluid such as ambient air flows into the compressor section where it is compressed before flowing into the combustor. The compressed working fluid is mixed with a fuel and burned within the combustor to generate combustion gases having a high temperature, pressure, and velocity. The combustion gases flow from the combustor and expand rapidly through the turbine section to rotate a shaft and to produce work. The combustion gases are then exhausted from the turbine section through an exhaust gas diffuser positioned downstream from the turbine section.

The exhaust gas diffuser typically includes an inner wall and an outer wall that is radially separated from the inner wall to form a flow passage through the diffuser. One or more struts extend between the inner and outer walls to provide structural support to the outer wall.

The struts are optimized for base load or full speed full load operation with a minimal angle of attack to minimize the drag, however the angle of attack increases due to increase turbine exit swirl during low load operations. As the angle of attack increases, the drag increases from which vortex shedding occurs creating sideways extending wakes. Such wakes may be unsteady and may create undesirable flow induced forces, vibration and associated noise. The induced forces and vibration can lead to structural fatigue failure reducing the structural life. Prior attempts to control vortex shedding from bluff bodies have included providing additional components such as spoilers, vortex generators, and trailing edge attachments with varying degrees of success and complexity. Therefore, an improved strut design would be useful in the art.

BRIEF DESCRIPTION OF THE INVENTION

Aspects and advantages of the invention are set forth below in the following description, or may be obvious from the description, or may be learned through practice of the invention.

One embodiment of the present invention is an exhaust gas diffuser having an inner wall that extends along an axial centerline of the exhaust gas diffuser. An outer wall is coaxially aligned with the inner wall. The outer wall is radially separated from the inner wall so as to define a flow passage therebetween. An airfoil shaped strut is disposed in the flow passage. The strut extends between the inner and the outer walls. The strut includes a leading edge and a trailing edge positioned relative to a direction of flow through the flow passage. The leading edge and the trailing edge are tapered between the inner wall to the outer wall in the direction of flow through the passage.

Another embodiment of the present invention is an exhaust gas diffuser having an inner wall that extends along an axial centerline of the exhaust gas diffuser. An outer wall having a first segment upstream from a second segment is coaxially aligned with the inner wall. The first and second segments are radially separated from the inner wall to define a flow passage through the exhaust gas diffuser An airfoil shaped strut is disposed in the flow passage. The strut extends between the inner wall and the first and second segments of the outer wall. The strut includes a leading edge and a trailing edge positioned relative to a direction of flow through the flow passage. The leading and trailing edges are tapered in the direction of flow through the passage. The leading edge is tapered from the inner wall to the first segment of the outer wall and the trailing edge is tapered from the inner wall to the second segment of the outer wall.

The present invention also includes a gas turbine including a compressor section at a forward end of the gas turbine, a combustor downstream from the compressor section, a turbine section downstream from the combustion section and an exhaust gas diffuser downstream from the turbine section. The exhaust gas diffuser includes an inner wall that extends along an axial centerline of the exhaust gas diffuser and an outer wall that is coaxially aligned with the inner wall. The outer wall is radially separated from the inner wall to define a flow passage therebetween. An airfoil shaped strut is disposed in the flow passage. The strut extends between the inner and the outer walls. The strut includes a leading edge and a trailing edge positioned relative to a direction of flow through the flow passage. The leading edge and the trailing edge are tapered from the inner wall to the outer wall in the direction of flow.

Those of ordinary skill in the art will better appreciate the features and aspects of such embodiments, and others, upon review of the specification.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including the best mode thereof to one skilled in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying figures, in which:

FIG. 1 illustrates a cross section side view of a known gas turbine;

FIG. 2 illustrates a simplified cross-section of an exhaust gas diffuser as shown in FIG. 1;

FIG. 3 illustrates a cross section side view of the exhaust gas diffuser taken at section line 3-3 as shown in FIG. 2, according to at least one embodiment of the present disclosure;

FIG. 4 illustrates a cross section side view of the exhaust gas diffuser taken at section line 3-3 as shown in FIG. 2, according to at least one embodiment of the present disclosure; and

FIG. 5 illustrates a cross section top view of an airfoil shaped strut taken along section line 5-5 as shown in FIG. 2, according to various embodiments of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to present embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. The detailed description uses numerical and letter designations to refer to features in the drawings. Like or similar designations in the drawings and description have been used to refer to like or similar parts of the invention. As used herein, the terms “first”, “second”, and “third” may be used interchangeably to distinguish one component from another and are not intended to signify location or importance of the individual components. In addition, the terms “upstream” and “downstream” refer to the relative location of components in a fluid pathway. For example, component A is upstream from component B if a fluid flows from component A to component B. Conversely, component B is downstream from component A if component B receives a fluid flow from component A.

Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that modifications and variations can be made in the present invention without departing from the scope or spirit thereof For instance, features illustrated or described as part of one embodiment may be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents.

Various embodiments of the present invention provide means for reducing aerodynamic losses across diffuser struts, and inner and outer wall surfaces due to flow separation of combustion exhaust gases flowing from a turbine of a gas turbine and into the exhaust gas diffuser at high tangential flow angles, particularly at part load operation of the gas turbine. The high tangential angles or “swirl” and the resulting flow separation reduce static pressure recovery, thereby reducing overall gas turbine efficiency.

The present disclosure provides for a plurality of airfoil shaped struts having a leading and a trailing edge where the strut is positioned within the exhaust gas diffuser with respect to a direction of flow through a fluid passage that extends through the exhaust gas diffuser. Generally, each strut has an aerodynamic profile that reduces the flow separation across the strut. In particular, the leading and the trialing edges of each strut is tapered from the inner to the outer wall in a common direction with respect to the direction of flow, thereby improving overall gas turbine performance in the presence of high swirl conditions. Although exemplary embodiments of the present invention will be described generally in the context of an exhaust gas diffuser incorporated into a gas turbine for purposes of illustration, one of ordinary skill in the art will readily appreciate that embodiments of the present invention may be applied to any exhaust diffuser and are not limited to a gas turbine exhaust gas diffusers unless specifically recited in the claims.

Typically, an industrial gas turbine is operated at base load speed, with the struts being fixed at a single position with a minimum swirl angle for providing maximum turbine efficiency. For example, during non-base load operation, the exhaust gas swirl angle at the inlet to the diffuser has a minimum value of about 60°. When this occurs, a suction side of the strut is exposed to the high swirl angle gases. This creates bluff bodies from which vortices are shed sideways from the strut, thereby creating the wakes.

For a straight strut, a single dominant wake shedding mode is created at a specific frequency which can lead to undesirable flow induced forces, vibration and associated noise. However, according to the present invention as disclosed herein, tapering or axially leaning the struts between the root and tip varies the flow separation along the radial span of the strut, thereby resulting in a varying amplitude and frequency. In this way, single dominant vortex shedding frequency is reduced.

Referring now to the drawings, FIG. 1 illustrates an example of a known gas turbine 10. As shown, the gas turbine 10 generally includes a compressor section 12 having an inlet 14 disposed at an upstream end of the gas turbine 10, and a casing 16 that at least partially surrounds the compressor section 12. The gas turbine 10 further includes a combustion section 18 having a combustor 20 downstream from the compressor section 12, and a turbine section 22 downstream from the combustion section 18. A shaft 24 extends generally axially through the gas turbine 10. The turbine section 22 generally includes alternating stages of stationary nozzles 26 and turbine rotor blades 28 positioned within the turbine section 22 along an axial centerline 30 of the shaft 24. A casing 32 circumferentially surrounds the alternating stages of stationary nozzles 26 and the turbine rotor blades 28. An exhaust gas diffuser 34 is positioned downstream from the turbine section 22.

In operation, air 36 or other working fluid is drawn into the inlet 14 of the compressor section 12 and is compressed. The compressed air flows into the combustion section 18 and is mixed with fuel to form a combustible mixture which is burned in a combustion chamber 38 defined within the combustor 20, thereby generating a hot gas 40 that flows from the combustion chamber 38 into the turbine section 22. The hot gas 40 rapidly expands as it flows through the alternating stages of stationary nozzles 26 and turbine rotor blades 28 of the turbine section 22.

Thermal and/or kinetic energy is transferred from the hot gas 40 to each stage of the turbine rotor blades 28, thereby causing the shaft 24 to rotate and produce mechanical work. The hot gas 40 exits the turbine section 22 and flows through the exhaust gas diffuser 34 across a plurality of airfoil shaped struts 42 that are disposed within the exhaust gas diffuser 34. The hot gas 40 flowing into the exhaust gas diffuser 34 from the turbine section 22 has a high level of swirl caused by the rotating turbine rotor blades 28, thereby resulting in flow losses through the exhaust gas diffuser 34 due to vortex shedding and flow separation as the hot gas 40 flows across the struts 42 and across inner walls of the exhaust gas diffuser 34.

FIG. 2 shows a simplified cross sectional downstream view of an exemplary exhaust gas diffuser 34 as may be used for the present invention. As shown, the exhaust gas diffuser 34 generally includes an inner wall 44 and an outer wall 46. The inner wall 44 extends generally axially along an axial centerline 48 of the exhaust gas diffuser 34. The inner wall 44 is generally annular shaped and may surround rotating components. For example, the inner wall 44 may surround or encase a portion of the shaft 24 of the gas turbine 10.

As shown in FIG. 2, the outer wall 46 is radially separated from the inner wall 44 with respect to a plane that extends perpendicular to the axial centerline 48 of the exhaust gas diffuser 34. The outer wall 46 generally surrounds the inner wall 44 to define a fluid flow passage 52 through the exhaust gas diffuser 34 between the inner and the outer walls 44, 46. In particular embodiments, the outer wall 46 is coaxially aligned with the inner wall 44. In certain embodiments, the outer wall 46 may be a double walled construction, with an inner wall 54 radially separated by an air space from an outer wall 56. The present disclosure is not limited to any particular size, shape, material, or other physical characteristics of the inner wall 44, the outer wall 46 and/or the outer wall walls 54, 56, except as recited in the claims.

FIG. 3 illustrates a cross sectional side view taken along line 3-3 of the exhaust gas diffuser 34 shown in FIG. 2, according to at least one embodiment, and FIG. 4 illustrates a cross sectional side view taken along line 3-3 of the exhaust gas diffuser 34 shown in FIG. 2, according to an alternate embodiment of the present disclosure. As shown in FIG. 3, the outer wall 46 includes a first segment 58 immediately downstream from the turbine section 22 and a second segment 60 positioned immediately downstream from the first segment 58.

In various embodiments, the first and the second segments 58, 60 are coaxially aligned with the inner wall 44. The first and second segments 58, 60 are radially separated from the inner wall with respect to a plane that extends perpendicular to the axial centerline 48 of the exhaust gas diffuser 34. The first and second segments 58, 60 are radially separated from the inner wall 44 so as to at least partially define the fluid flow passage 52 through the exhaust gas diffuser 34.

In particular embodiments, as shown in FIG. 3, the first segment 58 is flared radially outward with respect to the axial centerline 48 of the exhaust gas diffuser 34 from an inlet 62 of the exhaust gas diffuser 34 to an intersection point 64 with the second segment 60. The first segment 58 is flared at a first angle with respect to the axial centerline 48. The second segment 60 is flared radially outward from the intersection point 64 with the first segment 58 to a downstream end 66 of the second segment 60. The second segment 60 is flared at a second angle with respect to the axial centerline 48. In particular embodiments, the first angle is greater than the second angle. In alternate embodiments, as shown in FIG. 4, the outer wall 46 is flared radially outward with respect to the axial centerline 48 of the exhaust gas diffuser 34 from the inlet 62 of the exhaust gas diffuser 34 to an outlet 70 of the exhaust gas diffuser 34.

Referring back to FIG. 2, the struts 42 extend between the inner and the outer walls 44, 46 within the flow passage 52 defined therebetween. As shown, the struts 42 are spaced circumferentially around the inner wall 44. The struts 42 orient the inner wall 44 to the outer wall 46. In addition, the struts 42 may provide structural support between the inner and the outer walls 44, 46. The struts 42 are positioned relative to the direction of flow 68 of the hot gas 40 flowing from the turbine section 22 of the gas turbine 10. Each strut 42 generally includes a root portion 72 connected to the inner wall, and a tip portion 74 radially separated from the root portion 72. The tip portion 74 being connected to the outer wall 46. In the context of the present invention, the term “strut” includes any structure or supporting member that extends between the inner wall 44 and the outer wall 46.

FIG. 5 shows a cross-section of one of the struts 42 taken along section line 5-5 in FIG. 2, according to various embodiments of the present invention. As shown in FIG. 5, each strut 42 generally includes a leading edge 76 facing the direction of flow 68 of the hot gases 40 exiting the turbine section 22 of the gas turbine 10 shown in FIG. 1, and a trailing edge 78 downstream from the leading edge 76. An outer surface 80 extends between the leading edge 76 and the trailing edge 78 and the between root portion 72 (FIG. 2) and the tip portion 74 (FIG. 2) of each strut 42 to at least partially define the airfoil shape. Each strut 42 includes a chord length 82 that is defined between the leading edge 76 and the trailing edge 78 of the strut 42 with respect to the axial centerline 48 (FIG. 2) of the exhaust gas diffuser 34. In particular embodiments, as shown in FIGS. 3 and 4, the leading edge 76 and the trialing edge 78 extends between the root and the tip portions 72, 74 of each of the struts 42.

In particular embodiments, as shown in FIGS. 3 and 4, the leading edge and the trailing edge of each strut 42 is tapered from the inner wall 44 towards the outer wall 46 in the direction of flow 68 through the fluid passage 52 of the exhaust gas diffuser 34. For example, as shown in FIG. 4, the leading edge 76 at the root portion 72 of each strut 42 is disposed on the inner wall 44 upstream from the leading edge 76 at the tip portion 74 of the strut 42 with respect to the direction of flow 68 of the hot gases 40 flowing through the flow passage 52, and the trailing edge 78 at the root portion 72 of the strut 42 is disposed on the inner wall 44 upstream from the trialing edge 78 at the tip portion 74 of the strut 42 with respect to the direction of flow 68 of the hot gases 40 flowing through the flow passage 52.

In particular embodiments, as shown in FIG. 3, the leading edge 76 of each strut 42 is tapered from the inner wall 44 towards the first segment 58 of the outer wall 46 in the direction of flow 68 through the fluid passage 52, and the trailing edge 78 of each strut 42 is tapered from the inner wall towards the second segment 60 of the outer wall 46 in the direction of flow 68 through the fluid passage 52. For example, as shown in FIG. 3, the leading edge 76 at the root portion 72 of each strut 42 is disposed on the inner wall 44 upstream from the leading edge 76 at the tip portion 74 which is disposed on the first segment 58 of the outer wall 46, and the trailing edge 78 at the root portion 72 of the strut 42 is disposed on the inner wall 44 upstream from the trialing edge 78 at the tip portion 74 which is disposed on the second segment 60 of the outer wall 46. This configuration compensates for a shorter axial distance between a last stage of the turbine rotor blades 28 and the struts 42.

In particular embodiments, the leading edge 76 and the trailing edge 78 of each strut are parallel between the inner and outer walls 44, 46. In alternate embodiments, the leading edge 76 and the trailing edge 78 are non-parallel. For example, as shown in FIGS. 3 and 4, the leading edge 76 may be tapered from the inner wall 44 towards the outer wall 46 at a first angle 84 with respect to a plane that extends perpendicular to the axial centerline 48 of the exhaust gas diffuser 34 at a point wherein the leading edge 76 and the root portion 74 intersect, and the trailing edge 78 may be tapered from the inner wall 44 to the outer wall 46 at a second angle 86 with respect to a plane that extends perpendicular to the axial centerline 48 of the exhaust gas diffuser 34 at a point wherein the trailing edge 78 and the root portion 74 intersect. In various embodiments, as shown by the dashed lines in FIG. 4, the first angle 84 of the leading edge 76 may be greater than or less than the second angle 86 corresponding to the trailing edge 78.

This invention provides various technical benefits over existing exhaust gas diffusers. For example, tapering the leading and trailing edges of the struts from the inner to the outer walls in the direction of flow through the flow passage reduces blockages at the outer wall and unsteady pressure amplitudes within the exhaust gas diffuser, thus improving performance of the exhaust gas diffuser at full speed full load operation of the gas turbine. In addition, tapering the leading and trailing edges of the struts from the inner to the outer walls in the direction of flow through the flow passage reduces the chord length of the struts, thereby reducing separation losses across the struts during part load operation of the gas turbine.

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims. 

What is claimed is:
 1. An exhaust gas diffuser for a gas turbine, comprising: a. an inner wall extending along an axial centerline of the exhaust gas diffuser; b. an outer wall coaxially aligned with the inner wall, the outer wall being radially separated from the inner wall to define a flow passage therebetween; and c. an airfoil shaped strut disposed in the flow passage, the strut extending between the inner and the outer walls, the strut having a leading edge and a trailing edge positioned relative to a direction of flow through the flow passage, the leading edge and the trailing edge being tapered from the inner wall towards the outer wall in the direction of flow.
 2. The exhaust gas diffuser as in claim 1, wherein the leading edge and the trialing edge of the strut are parallel.
 3. The exhaust gas diffuser as in claim 1, wherein the leading edge and the trailing edge of the strut are non-parallel.
 4. The exhaust gas diffuser as in claim 1, further comprising a plurality of the airfoil shaped struts spaced circumferentially around the inner wall.
 5. The exhaust gas diffuser as in claim 1, wherein the airfoil shaped strut has a chord length defined along the axial centerline of the exhaust gas diffuser between the leading edge and the trialing edge of the strut, the chord length being constant between the inner and the outer walls.
 6. The exhaust gas diffuser as in claim 1, wherein the airfoil shaped strut includes a root portion connected to the inner wall and a tip portion connected to the outer wall, the leading and the trailing edges extending between the root portion and the tip portion.
 7. The exhaust gas diffuser as in claim 6, wherein the leading edge at the root portion is disposed upstream from the leading edge at the tip portion and the trailing edge at the root portion is disposed upstream from the trailing edge at the tip portion.
 8. An exhaust gas diffuser, comprising: a. an inner wall extending along an axial centerline of the exhaust gas diffuser; b. an outer wall having a first segment upstream from a second segment, the first and second segments being coaxially aligned with the inner wall, the first and second segments being radially separated from the inner wall to define a flow passage through the exhaust gas diffuser; and c. an airfoil shaped strut disposed in the flow passage, the strut extending between the inner wall and the first and second segments of the outer wall, the strut having a leading edge and a trailing edge positioned relative to a direction of flow through the flow passage, the leading and trailing edges being tapered in the direction of flow through the passage, the leading edge being tapered from the inner wall towards the first segment of the outer wall and the trailing edge being tapered from the inner wall towards the second segment of the outer wall.
 9. The exhaust gas diffuser as in claim 8, wherein the leading edge and the trialing edge of the strut are parallel.
 10. The exhaust gas diffuser as in claim 8, wherein the leading edge and the trailing edge of the strut are non-parallel.
 11. The exhaust gas diffuser as in claim 8, further comprising a plurality of the airfoil shaped struts spaced circumferentially around the inner wall.
 12. The exhaust gas diffuser as in claim 8, wherein the airfoil shaped strut has a chord length defined along the axial centerline of the exhaust gas diffuser between the leading edge and the trialing edge of the strut, the chord length being constant between the inner and the outer walls.
 13. The exhaust gas diffuser as in claim 8, wherein the airfoil shaped strut includes a root portion and a tip portion, the root portion being connected to the inner wall and the tip portion being connected to the first segment of the outer wall at the leading edge of the strut and to the second segment of the outer wall at the trailing edge of the strut.
 14. The exhaust gas diffuser as in claim 13, wherein the leading edge at the root portion is disposed upstream from the leading edge at the tip portion and the trailing edge at the root portion is disposed upstream from the trailing edge at the tip portion.
 15. A gas turbine comprising: a. a compressor section at a forward end of the gas turbine; b. a combustor downstream from the compressor section; c. a turbine section downstream from the combustor; and d. an exhaust gas diffuser downstream from the turbine section, comprising: i. an inner wall extending along an axial centerline of the exhaust gas diffuser; ii. an outer wall coaxially aligned with the inner wall, the outer wall being radially separated from the inner wall to define a flow passage therebetween; and iii. an airfoil shaped strut disposed in the flow passage, the strut extending between the inner and the outer walls, the strut having a leading edge and a trailing edge positioned relative to a direction of flow through the flow passage, the leading edge and the trailing edge being tapered from the inner wall towards the outer wall in the direction of flow.
 16. The gas turbine as in claim 15, wherein the leading edge and the trialing edge of the exhaust gas diffuser airfoil shaped strut are parallel.
 17. The gas turbine as in claim 15, wherein the leading edge and the trialing edge of the exhaust gas diffuser airfoil shaped strut non-parallel.
 18. The gas turbine as in claim 15, wherein the exhaust gas diffuser further comprises a plurality of the airfoil shaped struts spaced circumferentially around the inner wall.
 19. The gas turbine as in claim 15, wherein the airfoil shaped strut has a chord length defined along the axial centerline of the exhaust gas diffuser between the leading edge and the trialing edge of the strut, the chord length being constant between the inner and the outer walls.
 20. The exhaust gas diffuser as in claim 15, wherein the airfoil shaped strut includes a root portion connected to the inner wall and a tip portion connected to the outer wall, the leading edge at the root portion being disposed upstream from the leading edge at the tip portion and the trailing edge at the root portion being disposed upstream from the trailing edge at the tip portion. 